1. Field of the Invention
The present invention relates to a corona charger or a corona charging apparatus used, for instance, in an electro-photographic device. A corona charger is used for charging a photoconductive layer or a dielectric layer in such an electro-photographic device. In such a case, the characteristic of the corona charger generally requested is to charge the objective surface to be charged up to a predetermined potential as quick as possible. Furthermore, there is a requirement to charge the objective surface as uniform as possible. More particularly, the present invention relates to an improved corona charger having efficient and quick charging characteristics for obtaining a uniform charged surface.
2. Description of Prior Art
FIG. 1-A shows diagrammatically a traditional corona charger known as a "corotron" charger. This charger substantially comprises a corona wire 1 and a shield electrode 2 arranged to surround the corona wire 1. A high voltage of a desired potential is applied to the corona wire 1 by means of an electric voltage source 3. In this specification, the explanation will be made for a case that the corona wire is charged in positive polarity. The shield electrode 2 is connected to the ground potential together with a conductive layer 5 provided beneath the objective surface 4 to be charged and forming a back up electrode. A corona discharge is produced by an electric field produced between the high electric voltage of the corona wire 1 and the ground potential shield electrode 2. A part of the corona discharge current flows toward the objective surface 4 and charges it. While this traditional corona charger is able to make a high speed charging and it is said as high efficiency, this type of charger has a disadvantage in that it is difficult to charge the objective surface uniformly at a predetermined potential.
Namely, this type of charger is not equipped with a control means for the potential of the objective surface. Accordingly, the charged potential may vary by the voltage variation of the electric source 3 or by the variation of environmental condition such as moisture, atmospheric pressure or others. Furthermore, non-uniform charging of the objective surface arises when dust or rust is attached or existed on the corona wire 1.
In order to solve such disadvantage of the traditional device, a corona charger equipped with a grid electrode for controlling the charging potential had been proposed. This charger is called as "scorotron" and the essential diagram is shown in FIG. 1-B. This charger comprises a grid electrode 6 arranged at a surface where the corona ions are produced. The grid electrode 6 is applied with a predetermined bias voltage by a voltage source 7. In such a construction, if the objective surface is charged at a potential corresponding to said bias voltage, there will be no electric field potential between the grid electrode 7 and the objective surface 4 and the charging of the surface 4 is discontinued. Accordingly, by charging the objective surface 4 by producing a sufficient amount of the corona discharge, the objective surface can be charged uniformly at a predetermined potential which corresponds to said bias voltage. By this means, resulting of non-uniform charging or variation of charging potential due to voltage variation of the corona current source, variation of moisture, atmospheric pressure or by the presence of dust on the corona wire can be avoided. However, this device has a disadvantage in that a considerable amount of the corona ions are caught by the grid electrode and unable to pass the grid electrode and hence the efficiency of utilizing the corona ion current becomes very low.
By the above reason, this device is not suited for use in a high speed charging device. Moreover, since this type of device required to employ a high power corona discharge so that there was another disadvantage in that a poisonous gas such as ozone is produced in large amount.
FIG. 1-C shows behavior of the electric field and the corona ion stream around the grid electrode 6. As can be seen from this schematic illustration, a large amount of ion stream besides those passing through the opening 8 of the grid 6 shown by dotted lines is caught by the grid electrode 6 as shown by full line.
FIG. 1-D is another embodiment of a conventional device to improve abovementioned disadvantage. This embodiment comprises modified shaped grid electrode 9 applied with an insulating layer 10 on the surface facing to the corona wire 1. This construction of grid had been disclosed on the Japanese opened patent publication No. 137,345/77. In this construction when the corona discharge is started, the insulating layer 10 is also charged and an electric field from the insulating layer 10 toward the layer shaped grid electrode is formed. Due to presence of this electric field, the corona ion stream easily passes the gap between the grid electrodes and the utilizing efficiency of the corona ion current can be improved.
However, in this recently proposed construction, as the electric field for accelerating the passing of the corona ion stream between the grid wires 9 is produced by charging the insulating layer 10 by the corona ion stream, there are still disadvantages in that the charging of the objective surface becomes non-uniform due to a difference of the electric field for accelerating the passing of the corona ion stream in the following occasions.
(a) When the thickness of the insulating layer 10 is non-uniform.
(b) When there is non-uniformity in the corona ion stream produced from the corona wire 1.
It is obvious that this disadvantage may be avoided by using sufficiently long charging time so as to charge the objective surface 4 to have a same uniform potential with that applied to the grid electrode layer 9 even at the most weak charging point. However, by this practice, one of the object to improve the efficiency of the corona charger cannot be achieved.
Furthermore, in the conventional charging devices, since another electric field for the corona discharge exists between the corona wire 1 and the corona shield plate 2, a considerably large amount of the corona ion current flows towards the shield plate 2 which does not contribute the charging of the objective surface 4 and this causes also deterioration of the charging efficiency.